Delivery of molten metal, such as aluminum or aluminum alloys (collectively referred to herein as aluminum), to a casting furnace is a multi-step process. Initially, aluminum ingots may be melted in a melting furnace and the molten aluminum may then be dispensed from the melting furnace to a launder. In such an arrangement, the molten aluminum flows from the launder to a holding furnace (transfer furnace) where its temperature is preferably maintained prior to being introduced into a casting furnace.
While the molten aluminum is present in the transfer furnace, it may be degassed and filtered to remove absorbed oxygen and inclusions (e.g., metal oxides, dross) prior to being transferred to the casting furnace. A common method of transferring molten metal from a transfer furnace to a casting furnace is via a discharge trough that leads from the transfer (or holding) furnace to the casting furnace. According to this method, the molten metal flows by gravity from the trough into the casting furnace.
Generally, a casting furnace is located beneath a casting machine. Several mechanisms are currently employed to facilitate the transfer of molten metal from the discharge end of a holding furnace to a metal bath of a casting furnace. One common arrangement is a stopper rod box system. In this system, a stopper rod box is attached to the end of the transfer furnace discharge trough. The stopper rod box controls the flow of molten aluminum from the transfer furnace with a removable stopper rod. To transfer metal from the transfer furnace to the casting furnace, the stopper rod is removed from a hole in the bottom of the stopper rod box, allowing the gravitational flow of metal through the bottom of the box into an open air trough that connects to an open hole in the bottom platen of the casting machine. When the stopper rod is removed the metal falls to the surface of the metal bath in the casting furnace. To terminate flow, the stopper rod is inserted back into the hole in the stopper rod box. The hole in the platen is then shut with a flat plate and gasket to permit subsequent pressurization of the casting furnace —to move the metal up into the casting machine.
While the stopper rod box is a simple system with few moving parts, a significant amount of maintenance is required to prevent leakage of molten metal at the stopper rod. The components of the stopper box are located underneath molten aluminum, which makes them inaccessible during operations and difficult to maintain without shutting down the process. Poor maintenance can result in metal leaks at the discharge point and result in costly and time-consuming cleanup.
Alternatively, various pump configurations have been used to transfer molten aluminum from the transfer furnace to the trough that leads to the casting furnace. For example, Lindberg and Holimsey pumps are commonly employed and are well-known in the art.
In the case of a Lindberg pump transfer system, the pump is mounted in the discharge end of the transfer furnace. To transfer metal to the casting furnace, air pressure is applied to the top surface of the molten metal in the pump. The metal flows out of a channel running from the bottom of the pump to a discharge point above the pump housing and into an open air trough. From the trough, the metal follows a similar path to the casting furnace, namely cascading out of the trough into a hole in the platen and falling to the surface of the metal bath in the casting furnace.
In both the Lindberg and Holimsey pump transfer systems, metal is transferred from the transfer furnace to the casting furnace through the enclosed structure of the pump. While neither pump has any moving parts, the enclosed nature of these pumps makes periodic cleaning very time consuming. The transfer operations must be shut down to allow for disassembly of the pump for cleaning. Further, both pumps rely on a good quality seal during re-assembly to get a repeatable volume of metal transfer. In addition, the cascading of molten metal from either the stopper box or pump transfer systems, promotes the formation of oxides in the molten metal immediately prior to its introduction into the casting furnace.
When transferring molten aluminum from a transfer furnace to the casting furnace by means of either the stopper rod box assembly or a pump as described above, the volume of metal that is transferred is dependent on the level of metal in the transfer furnace. If the level of molten aluminum in the launder drops too dramatically, the amount of metal in the transfer furnace will be insufficient to provide an adequate volume of molten metal to the casting furnace.
It is desirable, therefore, to provide molten metal to the casting furnace on demand and substantially independent of the level (volume) of metal in the launder or transfer furnace so that casting may proceed in an efficient manner. Transfer via either a stopper rod box or current pumping technology exposes the molten metal to atmospheric oxygen unnecessarily. This exposure can contribute to the formation of oxides and inclusions within the molten metal supplied to the casting furnace. It would be desirable, therefore, to provide an apparatus and system that transferred an aliquot of molten metal to a casting furnace in a manner that substantially avoided contact with the atmosphere to thereby reduce the percentage of oxides and inclusions that are formed.
In addition, the temperature of the metal that is discharged from the transfer furnace can vary and may further contribute to undesirable properties of the finished cast metal. It would be desirable, therefore, to develop a system and apparatus for delivering molten metal to a casting furnace on demand in which the metal displays a substantially uniform temperature to help maintain or enhance the desirable properties of the finished cast metal.